This work ties together published experimental and theoretical results concerning the threshold condition for initiation of explosives during impact and/or penetration by projectiles or shaped charge jets. The shape of a projectile's tip, which may effect either pressure (if pointed) or duration (if rounded), has well known strong effects on the threshold velocity. A qualitative analysis is incorporated which satisfactorily relates these effects to results obtained with flat nosed rod impacts. Further, evidence is presented that even shaped charge jets display a significant nose shape effect, which adds complication to evaluating the response of explosives to jets. There is a strong, convincing evidence that for each impactor (effective) diameter, a certain threshold pressure is required for initiation. However, some (nominally) conflicting data exist and are discussed.